Lithium batteries have high voltages and high energy densities, and have enhanced stability compared to other batteries, such as a Ni—Cd batteries. Due to these advantages, lithium batteries are used as power sources for portable electric applications. However, since smaller, lightweight portable electric applications require power sources having high capacities, batteries having higher operating voltages, longer lifetimes, and higher energy densities than conventional lithium batteries are needed. Such batteries can be developed by improving the performance of various battery components. For example, battery properties are dependent on electrodes, electrolytes, and other battery materials. In particular, electrode properties are dependent on electrode active materials, current collectors, and binders which bind the electrode active materials and current collectors. The amount of lithium ions bound to an active material depends on the amount and type of active material. Consequently, batteries having high capacities can be produced using more active material and using an active material having large intrinsic capacity. In addition, when a binder provides a strong binding force between the active materials or between the active materials and the current collector, electrons and lithium ions move smoothly within the electrode, thereby decreasing the inner resistance of the electrode and enabling relatively high charge/discharge rates.
High capacity batteries require composite electrodes including metal and graphite. The active material of such a composite electrode considerably expands and contracts during charging and discharging. Accordingly, in addition to good binding force, the binder should have good elasticity and recovery properties in order to maintain the original binding force of the binder and the electrode structure even after expansion and contraction.
When fluorinated polyvinylidene fluoride (PVDF) based polymers are used as the binder material, the material is dissolved in a solvent such as N-methyl-2-pyrrolidone. PVDF based polymers have strong binding forces. However, the PVDF based polymer only expands by about 10%. Consequently, a large amount of the PVDF based polymer is needed to obtain a sufficient binding force. In addition, the PVDF based polymer is dissolved in an organic solvent before use, thereby complicating the manufacturing process.
Styrene-butadiene rubber (SBR) can also be used as a binder material and it has good elasticity. However, the binding force of SBR is very weak, resulting in changes in the structure of the electrode after a number of charge/discharge cycles and the reductions in battery capacity and lifetime.
Accordingly, there is a need to develop a waterborne binder having good elasticity and binding properties in order to improve charge/discharge properties of batteries.